This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9902629-6 filed in Sweden on Jul. 8, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to a balun circuit according to the preamble of claim 1.
High frequency electric signals can be transmitted in two often occurring ways, namely balanced and unbalanced. In balanced transmission there is used two conductors in which electric currents are constantly in antiphase. Unbalanced transmission, on the other hand, uses only one signal conductor and the signal (the current) is returned via earth. The balanced transmission is differential in nature and thus less sensitive to disturbances and interference than the unbalanced transmission.
Balanced and unbalanced transmissions are often mixed in radio systems. It is therefore necessary to enable a balanced signal to be converted to an unbalanced signal, and vice versa, with the smallest losses possible. Baluns are used to this end.
The properties of a balun circuit depend on impedance difference and phase difference for odd and even modes in the high frequency electric signal.
A typical balun is the so-called Marchand balun. The Marchand balun includes four xcex/4-waveguides coupled in pairs. The Marchand balun gives a 4:1 transformation, which means that a differential impedance applied to the balun input shall be four times greater than an impedance desired on an output of the Marchand balun.
This is achieved by connecting a matching network to the actual Marchand balun. In the majority of situations in which baluns are used in practice, the impedance on an unbalanced output shall be 50xcexa9. When the Marchand balun is used, the impedance on the balun input shall thus be transformed to 200xcexa9 via said matching network.
When using the Marchand balun, a transformation effected with the aid of said matching network will have a very narrow band and be sensitive to scattering in both load impedance and in the individual components in the matching network, which constitutes a problem. This solution also results in pronounced scattering in output power from the balun, which also constitutes a problem.
One object of the present invention is to provide a balun circuit which will at least reduce the aforesaid problems.
This object is achieved in accordance with a first aspect of the present invention with a balun circuit according to claim 1.
One advantage afforded by the inventive balun circuit is that certain variations in implementation can be allowed without experiencing excessive reduction in the balun circuit output power.
Another advantage afforded by the inventive balun circuit is that all ports on the circuit can be biased in a simple manner with the aid of a minimum of components to this end.
Another advantage afforded by the inventive balun circuit is that it can be implemented in a comparatively compact form on or in a substrate.
The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings.